In recent years, as mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having a high energy density and a high discharge voltage, on which much research has been carried out and which is now commercially and widely used.
Also, as concern about environmental problems has increased, much research has been carried out on electric vehicles and hybrid electric vehicles, which can replace existing vehicles, such as gasoline vehicles and diesel vehicles, using fossil fuel, which is one of the principal environmental pollution sources. A nickel-metal hydride secondary battery is mainly used as a power source for the electric vehicles and the hybrid electric vehicles. In recent years, however, the use of lithium secondary batteries, which have a high energy density and a high discharge voltage, as the power source for the electric vehicles and the hybrid electric vehicles has been attempted. Some of the lithium secondary batteries have now entered a commercialization stage.
The lithium secondary battery is constructed in a structure in which an electrode assembly, including cathodes each having an active material applied to its cathode current collector, anodes each having an active material applied to its anode current collector, porous separators disposed respectively between the cathodes and the anodes, is impregnated with a non-aqueous electrolyte including lithium salt. Lithium cobalt oxide, lithium manganese oxide, or lithium nickel oxide, or lithium composite oxide is used as the cathode active material. A carbon material is used as the anode active material.
A high-power, large-capacity battery system is required in order for the lithium secondary battery to be used as a power source for electric vehicles and hybrid electric vehicles. High-rate charge and discharge characteristics are required for such a high-power, large-capacity battery system.
For a hybrid electric vehicle, the operations of a battery system and an engine of the vehicle are changed depending upon the running conditions of the vehicle in order to minimize the use of fuel while improving the operation efficiency of the vehicle. For example, when the vehicle is running at a normal speed or running down an incline, the engine is not used, and the battery system is operated, whereby a high-rate discharge characteristic is required. When the vehicle is running in an accelerating fashion or running up an incline, the engine is mainly operated. In this case, kinetic energy is converted into electric energy, by which the battery system is charged, whereby a high-rate charge characteristic is required. Consequently, it is most ideal that both the charge characteristic and the discharge characteristic of the battery system are superior.
However, the charge characteristic and the discharge characteristic are complementary to each other. For this reason, when an electrode active material has a relatively superior charge characteristic, the electrode active material has a relatively inferior discharge characteristic. On the other hand, when an electrode active material has a relatively superior discharge characteristic, the electrode active material has a relatively inferior charge characteristic. For this reason, it is difficult for a secondary battery, including such electrode active materials, to exhibit a superior charge characteristic as well as a superior discharge characteristic. For example, lithium iron phosphorus oxide (an olivine compound) exhibits a charge characteristic superior to a discharge characteristic. Accordingly, a lithium secondary battery including the lithium iron phosphorus oxide as a cathode active material also exhibits a charge characteristic superior to a discharge characteristic. On the other hand, lithium titanium oxide (of a spinel crystal structure) exhibits a discharge characteristic superior to a charge characteristic. Accordingly, a lithium secondary battery including the lithium titanium oxide as a cathode active material also exhibits a discharge characteristic superior to a charge characteristic.
In conclusion, currently developed battery systems do not simultaneously exhibit a superior charge characteristic and a superior discharge characteristic. In this connection, lithium secondary batteries having different charge and discharge characteristics may simply combined to construct a battery system. However, it has been proven that such combination of the lithium secondary batteries provides a desired effect.